The field of data communications typically uses a device, such as a modem, to convey information from one location to another. Digital Subscriber Line (DSL) technology now enables modems to communicate rapidly large amounts of data. Modems communicate by modulating a baseband signal carrying digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a signal space constellation, encoding the information to reduce errors and improve throughput, and transmitting the information over a communication channel. The constellation can include both analog and digital information or often merely digital information.
At a receiver, the constellation is received, equalized and decoded in accordance with techniques that those skilled in the art will appreciate.
In the above mentioned communications system, a control device, i.e., one that is located at a telephone company central office, connects over the communication channel to a plurality of remote devices typically located at a customer residential or business location. This topography is known as a multipoint communication environment because one control device is communicating with a plurality of remote devices over a single communication channel. The communication technique between the control modem and the remote modems is generally half duplex in nature, meaning that only one device may transmit at any particular time. In order to establish a communications connection between the central office modem and any of the remote modems, a lengthy preamble, which allows the modems to synchronize at the start of each message is required. Existing techniques such as carrierless amplitude/phase modulation (CAP) and discrete multitone (DMT) modulation allow modems to transmit simultaneously between only two modems at a time. In a multipoint environment, greater circuit efficiency is possible because of the ability to connect multiple modems to the same communication channel.
In a 4 wire communications environment, the control device can transmit continuously so that outbound preambles (from central office to remote location) are unnecessary, thus assuring remote device synchronization. In a 2 wire multipoint environment, as contemplated by the present invention, when a remote device transmits, it disrupts the outbound control signal from the control device preventing all remote devices connected to the communication line from maintaining synchronization with the control device. It is desirable to allow all remote devices to continuously maintain synchronization with the control device in order to significantly reduce the training preamble necessary to establish a communication path between a remote device and the control device. A technique to accomplish this continuous synchronization is described in commonly assigned copending U.S. patent application Ser. No. 08/936,415, titled "PILOT TONE SYSTEM AND METHOD TO ALLOW CONTINUOUS SYNCHRONIZATION IN MULTIPOINT NETWORKS" filed on Sep. 25, 1997, and is hereby incorporated by reference. Part of the system disclosed in the aforementioned application requires the use of highly accurate digital to analog (DAC) converters to supply an analog control signal to a voltage controlled crystal oscillator (VCXO). It would be desirable to provide the modem synchronization function described, at a reduced level of complexity and cost by simplifying the manner in which the analog control signal is developed and supplied to the VCXO.